Inflatable dielectric tube antenna



Sept- 27, 1966 K.1KRATH ETAL INFLATABLE DIELECTRIC TUBE ANTENNA Filed July 2, 1964 United States Patent Office Patented Sept. 27, 1966 3,276,025 INFLATABLE DIELECTRIC TUBE ANTENNA Kurt Ikrath, Elberon, and Wilhelm A. Schneider, Fair Haven, NJ., assignors to the United States of America as represented by the Secretary of the Army Filed July 2, 1964, Ser. No. 380,106 8 Claims. (Cl. 343-757) This invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The present invention relates to antennas and more particularly to an inflatable antenna which achieves high directivity without large metallic reflectors and which can lbe dellated to achieve portability and variation `of beamwidth. These features make the antenna particularly advantageous for military field use. Brie-ily stated, the antenna comprises a hollow tube or cylinder of ilexible dielectric material for example, plastic. `One end of the tube is sealed in airtight relation to a source of micro- Wave radiation. The other or free end of the flexible plastic tube is sealed or provided with a valve for controlling the pressure therein. Upon inllation the tube comprises a lens or dielectric antenna which focuses or refracts the microwave radiation into a narrow beam in the direction of the axis of the tube. When deflated, the tube can be rolled up for ease of transportation.

It is therefore an object of this invention to provide a novel and useful microw-ave antenna.

It is another object of the invention to provide a directive antenna which may be easily transported.

A fur-ther object of the invention is to provide a directive antenna which is inflatable, portable, and can be varied in beamwidth.

lOther objects and advantages of the invention will become apparent from the following detailed description and drawings, in which:

FIG. 1 is a cross-section of the antenna vtaken along the axis of the flexible tube in the inflated state and FIG. 2 shows the antenna in the deflated state.

In FIG. l the intlated flexible dielectric tube 3 is shown attached at its left end to the circular mouth of a microwave horn radiator 4. The microwave horn 4 is one terminal of a waveguide system, the other terminal of which may lbe a transmitter or a receiver, depending on whether the antenna is used for transmitting or receiving. The mouth of horn 4 may be conveniently mounted in an aperture 11 in the side wall 8 of a truck, tank or other mobile vehicle. The dielectric tube 3 is preferably cylindrical in shape and may have a hemispherical rigid cap 9 at its outer end. The tube is inflated with sulcient pressure to render it str-aight and rigid by means of pump 6 which has an outlet 5 which communicates with the interior of the horn 4. An air seal which is substantially transparent to microwave energy constrains the pump output to the horn and `the flexible tube. The pump inlet is shown at 7. The hemispherical cap 9 may include a bleeder valve 10 which allows the air inside the tube 3 to slowly escape, in which case the pump 6 would operate continuously to maintain the proper pressure within the hollow tube 4. The advantage of this arrangement is that the pump can be arranged to supply a continuous stream of heated air to the flexible tube to prevent icing. The source of the heated air may be a heater connected to the pump inlet 7 or may simply be the warm air from inside the vehicle. Further, by directing the axis of the bleeder valve 10 away from the axis of the cylindrical tube 3, the reaction of the air escaping therefrom will cause the bleeder valve and the end cap 9 to move in a circular path and the tube 3 will consequently move in conical fashion. By this means the antenna is capable of conical scanning. Alternatively, the tube may be airtight and a check valve inserted in the pump outlet 5. In this event pump need not be operated continuously.

The present antenna may be used with other types of feeds, for example, instead of a waveguide horn as illustrated the inflatable tube may be used in conjunction with a helical type antenna operated in the axial mode so that the radiation therefrom is along the axis of tube 3. Another possibility is a dipole `fed by a coaxial line, in which case a balun would -be necessary to convert the unbalanced output o-f the coaxial line to balanced form for application to the dipole. Generally, the inflatable tube 3 will focus or retract any source of radiation directed generally `along its longitudinal axis. The dielectric tube 3 should be at least one free space wavelength in diameter. The length of the tube ywill depend on the thickness of the tube walls and on the dielectric constant of the tube material. The energy which flows within and near the dielectric material will be reduced in velocity compared to that which travels through the air, the .amount of such reduction being proportional to the dielectric constant of the material. For optimum focusing and narrowest beamwidth, the tube length should be such that the phase difference between energy traveling within and outside the dielectric material is at the free or outside end of the dielectric tube. For most flexible dielectrics in common use, this means that the tube length should be from 7 to 12 wavelengths of the operating frequency.

An experimental antenna constructed of polyvinyl plastic was 2 ft. long and 11/2 in. in dia-meter with a wall thickness of approximately 1/16 in. At an operating frequency of 8000 mc., the half power beamwidth of this antenna was 9 in the inflated state and 27 in the dellated, rolled up state.

FIG. 2 shows the antenna -of FIG. 1 deflated and rolled up. In this condition the antenna can be easily transported. Also, the antenna can be used in `the deflated rolled up condition in which case the bearnvvidth will be greater than in the inflated state by `a factor of up to 3 to l.

While the invention has been described in connection with an illustrative embodiment, it Iis obvious that the inventive concepts disclosed herein are of general application and hence the invention should be limited only by the scope of the appended claims.

What is claimed is:

1. A directional antenna comprising a hollow, flexible, cylindrical tube of dielectric material, one end of said tube being fixed in airtight relat'io-n to a source of micro- Wave radiation, a hemispherical end cap sealin-g the othe end of said tube, a bleeder valve in said end cap, and a continuously operable air pump having its output communicating with the interior of said tube.

2. The a-pparatus of claim 1 wherein said bleeder valve discharges air in such a direction as to cause said other end of said tube to move in a circular path, thereby providing conical scanning.

3. An inflatable, portable antenna comprising, a Waveguide horn with its mouth mounted lin an aperture of a vehicle side wall, one end of a hollow tube of flexible dielectric material sealed in airtight relation to said mouth of said horn, the other end -of said tube including -a bleeder valve, a pump hav-ing an outlet communicating lwith the interior of said hollow tube and adapted to inllate 'said tube, the `diameter orf said tube being approximately one free space wavelength and the length of said tube from 7 to 12 free space wavelengths of the operating frequency.

4. The 4antenna of claim 3 wherein said pump is adapted to supply a continuous stream of heated air to maintain said tube inflated and to de-'ice said tube.

5. The antenna of claim 3 in which said bleeder valve is so directed that the reaction of the air escaping therefrom causes said tube to move in conical fashion.

`6. A directional microwave antenna comprising, a horn type radiator, one end of a hollow tube of flexible plastic sealed in airtight relation to the circular mouth of said radiator, the other end of said hollow tube being sealed, and means to iniiate said hollow tube with sufficient air pressure to render it straight and rigid, said tube being approximately one free space wavelength in diameter and from 7 to 12 free space wavelengths inaxial length.

7. An inflatable directional microwave antenna comprising, a exible hollow tube of dielectric material xedly attached at one end to a source of microwave radiation,

and means to inflate said tube with Suicient air pressure to render it straight and rigid,

8. An inflatable directional antenna comprising, a flexible hollow tube of dielectric material xedly attached at one end of a source of microwave radiation, a pump having an outlet communicating with the interior of said tube and a bleeder Valve at the other end of said tube, said bleeder valve being directed so that the reaction of the air escaping therefrom causes said tube to move in a conical fashion.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner. M. NUSSBAUM, Assistant Examiner. 

8. AN INFLATABLE DIRECTIONAL ANTENNA COMPRISING, A FLEXIBLE HOLLOW TUBE OF DIELECTRIC MATERIAL FIXEDLY ATTACHED AT ONE END OF A SOURCE OF MICROWAVE RADIATION, A PUMP HAVING AN OUTLET COMMUNICATING WITH THE INTERIOR OF SAID TUBE AND A BLEEDER VALVE AT THE OTHER END OF SAID TUBE, SAID BLEEDER VALVE BEING DIRECTED SO THAT THE REACTION OF THE AIR ESCAPING THEREFROM CAUSES SAID TUBE TO MOVE IN A CONICAL FASHION. 